Method of regulating an electric glow discharge and discharge vessel therefor



Jan. 11, 1966 B. BERGHAUS ETAL 3,228,809

METHOD OF REGULATING AN ELECTRIC GLOW DISCHARGE AND DISCHARGE VESSELTHEREFOR Original Filed Dec. 8, 1954 6 Sheets-Sheet 1 H/np. 30 40000P=aj7mm Hg T= 505 6 25 I I D 1 a /00 200 300 400 600 600 INVENTORSATTORNEYS w a a m 8 S 2 u 2 t E e 36 m S Jan. 11, 1966 B. BERGHAUS E ALMETHOD OF REGULATING AN ELECTRIC GLOW DISCHAR AND DISCHARGE VESSELTHEREFOR Original Filed Dec. 8, 1954 INVENTORS fiemizam flezylmusATTORNEYS Jan. 11, 1966 B. BERGHAUS ETAL 3,228,809

METHOD OF REGULATING AN ELECTRIC GLOW DISCHARGE AND DISCHARGE VESSELTHEREFOR Original Filed Dec. 8, 1954 6 Sheets-Sheet 5 INVENTORS maBernhard Berglmus Ham liwelc .Fz 5:

ATTORNEYS Jan. 11, 1966 B, BERGHAUS ET AL 3,228,809

METHOD OF REGULATING AN ELECTRIC GLOW DISCHARGE D DISCHARGE VESSELTHEREFOR Original Filed Dec. 8,

INVENTORS Berzzfiard Berylzaus HMS Bucek ATTORNEYS Jan. 11, 1966 B.BERGHAUS ETAL 3,228,809

METHOD OF REGULATING AN ELECTRIC GLOW DISCHARGE AND DISCHARGE VESSELTHEREFOR Original Filed Dec. 8, 1954 6 Sheets-Sheet 5 Volt. Span/2mg USUO/TZ I INVENTORS Berzzlzard Berglzaufi H0120 Bade/c BY m0 ATTORNEYS BBERGHAUS ETAL 3, ,8 METHOD OF REGULATING AN ELECTRIC GLOW DISCHARGE ANDDISCHARGE VESSEL THEREFOR 6 Sheets-Sheet 6 Jan. 11, 1966 Original FiledDec. 8, 1954 INVENTORS ATTORNEYS United States Patent METHOD OFREGULATING AN ELECTRIC GLOW glSCHARGE AND DISCHARGE VESSEL IHERE-Bernhard Berghaus and Hans Bucek, Zurich, Switzerland, assignors toElelttrophysikalische Anstalt Bernhard Berghaus, Nadnz, Liechtenstein, acorporation of Liechtenstein Original application Dec. 8, 1954, Ser. No.473,895. Divided and this application Sept. 24, 1962, Ser. No. 225,714

9 Claims. (Cl. 148-16) The present application is a continuation-in-partof application No. 627,685 filed December 11, 1956, now abandoned, whichin turn was a continuation-in-part of application Serial No. 579,935,filed April 25, 1956, now abandoned, which in turn was a division of ourapplication Serial No. 473,895, filed December 8, 1954, and nowabandoned.

The present invention relates to a process for the regulation ofelectric glow discharges in discharge chambers for carrying out variousoperations on materials subjected to the action of the discharges.

In the application of glow discharges in a vessel having a reducedpressure for the purpose of performing metallurgical, chemical or othertechnical processes, metallic discharge vessels are commonly employed.Such processes always require that the material treated be at anelevated temperature. In electric glow dis charges of this type, thematerial treated, such as a metal workpiece to be improved at itssurface, has a continuous or temporary negative voltage applied to it sothat the effective cathodic glow discharge may cover the surfaces of theworkpiece involved in the process as completely as possible. The supplyof electric energy required to the material treated is usually effectedvia one or several current lead-ins incorporated in the metallic wallsof the vessel and having an insulated internal lead which conducts acathodic potential, continuously in direct current operation andintermittently in alternating current operation.

In the operation of such discharge vessels the wellknown difliculty hasarisen that not only the material being treated but all other metalmembers having a cathodic potential, e.g. the internal lead of thecurrent lead-in, are covered by a surface glow and thus take part in thedischarge process. This causes, on the one hand, unnecessary heating andloss of energy in the members involved, such as the holders for thematerial treated and, on the other hand, destructive reactions,particularly at the current lead-ins, where especially heavy damagetakes place at the points of contact between the internal lead and theinsulating material facing the interior of the vessel. This well-knownphenomenon, which substantially reduces the life of such lead-ins, hasmade it advisable to use a system of narrow gaps between the internallead and the insulator in order to protect the point of contact betweenthe internal lead and the insulator situated at the bottom of the systemof gaps against the action of the destructive glow discharge, sinceexperience has shown that glow discharges are largely inhibited orcompletely eliminated in a sufficiently narrow gap. The favorable effectof such gap systems has resulted in various, usually rather complicated,designs of insulated current lead-ins, in which narrow protective gapsare provided between the internal lead on the one hand and the insulatorand possible metal parts connected with the insulator holder and thehousing on the other.

The use of protective gaps around the internal lead of the insulatedcurrent lead-ins has so far constituted the only known means ofeliminating the destructive glow discharge at the sensitive points ofthe lead-in. Such lead-ins have proved perfectly useful, in particularin the operation of discharge vessels having a very low gas pressure ofnot more than a few millimetres of mercury. However, it often occursthat a relatively high temperature, e.g. over 500 C., of the material tobe treated is desirable in glow discharge processes, which requires acorrespondingly large supply of energy. If lower gas pressures below 1mm. mercury were employed, a relatively high operating voltage of farabove 1000 volts would be necessary, which would not be advantageousowing to insulation difficulties and the complicated current lead-ininstallations. It is accordingly more advantageous to operate at higherpressures, such as in the range of 5 to 20 mm. mercury, to obtaintemperatures up to 1000 C. by the use of much higher discharge currentdensities at far lower operating voltages, e.g. between 400 and 1000volts.

At gas pressures above 5 mm. mercury the protective gap system losesmuch of its effectiveness for at gas pressures of only mm. a gap of .5mm. width cannot substantially prevent glow discharges from entering thegap, and the reduction of the gap width involves difliculties in designand operation. While current lead-ins have been provided in which gapsystems having a width of .2 mm. can be controlled by means ofparticular centering measures, such narrow gaps cannot preventdestructive glow discharges from entering the gap at pressures above 10mm. mercury. Accordingly, in the pressure range of above approximately 1mm. mercury, other suitable measures are desirable in order to protectthe current-lead-ins against the destructive influence of-highenergyglow discharges.

The present invention relates to a method of performing suchglow-discharge processes by which it is possible largely to circumventthe undesirable glow discharges, in particular at the current lead-ins.

It is a characteristic feature of the invention that in order to preventa discharge from having an impact on the lead-ins beyond the maximumadmissible value, the atmosphere used for the treatment, the propertiesof the electrodes and the geometrical arrangement of the elec trodes inthe receptacle are so adapted to the surfaces involved in the processthat the maintenance of a discharge thereon is favored and in the endcondition of the discharge a predetermined division of the output infavor of certain surfaces is produced. In this case the specific outputon these surfaces is predetermined, while as regards at least a part ofthe other constructional elements carrying voltage the specific outputis smaller.

As regards the atmosphere of the treatment, the kind of the gas and thegas pressure are determining factors, whilst as regards the propertiesof the electrode, it is the electric conductivity and the electricbehavior of the boundary layer between surface and gas that areimportant. The geometrical arrangement of the electrodes in thereceptacle comprises the surface ratios (electrode to counter-electrode,or to constructional support, or to wall), the distances (betweenelectrode and electrode, and between electrode and wall) as well as theconditions of shapes (total surface, surface parts facing one another).lso of importance are a limiting voltage and a limiting gas pressurewhich will be explained in detail below.

By means of the method according to the invention it is possible whilemaintaining a predetermined density of energy at the surfaces involvedin the process, to create a state of discharge and, in so doing, not togo above that limiting voltage at the electrodes or below that limitinggas pressure at which the high-energy glow discharge covers all the atleast intermittently negative structural members. If discharge isconducted with a voltage lower than the limiting voltage and a pressurehigher than the limiting gas pressure while a suitable discharge spaceresistance is chosen at the surfaces involved in the process, the ,glowdischarge will tend to withdraw from the current lead-ins thus ensuringthat the voltage carrying members of the current lead-ins will berelieved of energy in favor of the surfaces involved in the process.

The discharge vessel according to the present invention for theperformance of the process is provided with Walls formed largely ofmetal, and having at least one insulating current lead-in and at leastone additional supply point, holders for the material to be treated andcounter-electrodes. The discharge vessel is characterized by the factthat the areas assigned to the process surfaces are arranged anddimensioned in area in such a manner relative to the distance and areaconditions at the point of entry of the internal lead of the currentlead-in which is securely connected to the material to be treated andthe adjacent metal parts connected to the other points of supply, as tocreate a condition which favors the discharge in the vicinty of theprocess surfaces.

, Various embodiments of the invention are hereinafter described withreference to FIGURES 1-18 of the accompanying drawing, in which:

FIGS. 1 and 2 are each diagrammatic representations of a dischargereceptacle and are intended to explain the method;

FIG. 3 is a diagram showing the starting process;

FIG. 4 is a current/voltage characteristic for a glow dischargeaccording to the invention;

FIG. 5 is a diagram showing the lowering of the cycle;

FIG. 6 is a diagrammatic representation of a further dischargereceptacle;

FIG. 7 is a diagrammatic illustration on an installation and dischargereceptacle for the treatment of steel tubes;

FIG. 8 is a longitudinal section of a leading-in insulator for thedischarge receptacle shown in FIG. 7;

FIG. 9 is a longitudinal section of a further embodiment of a dischargevessel in diagrammatic view;

FIG. 10 is a cross section of the discharge vessel according to FIG. 9along line AA;

FIGS. 11 and 12 are longitudinal sections of the cath- 'od ic currentlead-in of the discharge vessel according to FIG. 9;

FIGS. 13 and 14 each show a further embodiment of discharge vessels;

FIG. 15 shows the current/voltage characteristic of a prior art glowdischarge and a glow discharge of the present invention;

FIGS. l6l8 are diagrams for the inherent resistance of two kinds of glowdischarges.

The method'according to the invention is based on the knowledge,consolidated by many years of experimentation, that the hightransformation of energy required for the carrying out of metallurgicaland chemical processes by means of glow discharges on an industrialscale can be attained in a discharge vessel in the case of continuousoperation only when a very substantial condition of discharge can bereached and maintained. This is clear in View of the fact that atransformation of energy on the surfaces participating in the process,of the order of 3050 Watts per sq. cm. and 20,000 watt per treatedworkpiece, is required and is obtainable in continuous operation forseveral days, and can be maintained without difiiculty. The technicalprocesses which can be carried out by means of glow discharges include:the diffusion of substances into metal surface, for instanceincorporation of nitrogen, boron, silicon, tungsten, etc., as well asprocesses of a chemical character, such as reductions, hydrations,polymerization, etc.

In such large-scale industrial glow discharge processes, temperatures of300 C. and above must be produced on the material to be treated. In viewof these temperatures,

the high energy transformation and the large workpiece or quantities ofmaterial to be treated, only discharge vessels built of iron or othersuitable materials having large dimensions can be used. The currentlead-ins through the walls of such vessels, since they are often used tohold the material to be treated, are very sturdily built while thecounterelectrodes-which are usually connected with a second currentlead-in-havc a substantial area or length.

With such spatial dimensions and the 'high capacities between thestructural members and the large dimensions of the vessel, the simpleprinciples of glow discharges established with experimental apparatususually formed of glass are of little importance since they are stronglyinfluenced and largely overlapped by secondary effects. While conditionsin the range of the cathodic glow edge are more or less in accord withthe known rules, conditions obtained in the remainder of the dischargespace are entirely different owing to the usually extended paths for thecharge carriers. The application of the present process is expresslylimited to glow discharge vessels and electrodes having largedimensions, i.e. to glow-discharge equipment beyond the range of radioand glow tube engineering.

Obviously, such a treatment can be reliably carried out only when it ispossible to concentrate the powerful glow discharge upon the surfaces tobe treated or otherwise taking part in the technical process, and uponthe layer of gas directly adjacent thereto, whether these surfaces aremetal workpieces, or only supports for the substances to be treated.This concentration of the energy transformation upon definite surfacesor gas layers is firstly required for reasons of economy, since only onthe surfaces that participate in the process does an energytransformation take place which is useful and contributes to thecarrying out of the process, whilst the energy transformation that takesplace at other points in the discharge receptacle, such as theleading-in insulators, the work-piece supports, the surfaces of theworkpiece which have not to be treated, walls, etc., represents a lossin output. In ad dition, a discharge leak, for instance from the oneelectrode to the conducting wall and from there back to the otherelectrode reduces efliciency and must be avoided. Further for reasons ofsafe operation, it is absolutely essential to divide up the output withat least a partial concentration of the energy transformation on thesurfaces participating in the process, since--as already mentionedab0vethe other voltage carrying constructional parts, and moreparticularly the leading-in insulators, ought not to receive duringcontinuous operation the impact of more than a definite relatively lowdischarge. These technical data thus compel the output of the dischargeon the energy-favored surfaces to be increased while at the saine timerelieving other surfaces, more particularly the leading-in insulators,in order to enable technical processes of the above-mentioned nature tobe carried out at all safely.

As is well known, the current/ voltage characteristic 65 (see FIG. 15)of a gas discharge receptacle operated, by way of example, withdirect-current voltage reveals a socalled stabilizing range x at which,while the electrode voltage remains constant, the discharge receptaclecan take currents of varying intensity. This stabilizing effect iscaused by the progressive glow covering of the electrode connected ascathode. It is thus not normally possible to increase the energytransformation in the discharge receptacle by raising the voltage at theelectrodes when there is a predetermined, only partial glow covering ofthe electrode, since any increase in voltage results first of all in anenlargement of the glow covering. If with such a normal glow discharge,the current is increased more and more, said discharge will finallyreach the socalled abnormal range y of the characteristic and thecathode drop in front of the electrode ccna nected as the cathode rises,which, with a further increa rr nt,

however, leads to the production of a very small area of impact on thiselectrode and causes beyond the range y in FIG. another form ofdischarge (point 67) known as an electric are which is quite unusablefor the present purpose. In order, therefore, while favoring thesurfaces to be treated in respect of energy, to increase the energytransformation in the discharge receptacle and the specific output onthese surfaces to practically any extent desired, completely differentdischarge condition must be created which represent essential featuresof the method according to the invention. The hitherto known glowdischarge according to the voltage/ current characteristic 65 of FIG. 15is not suitable for producing the desired high energy density at thesurfaces to be treated because beyond the range y the glow dischargeturns up to an arc discharge. Specific output as herein used alwaysrefers to the output transformed per unit of area, or the outputtransformed pre unit of volume in the gas layer.

The method according to the invention requires special measures whichassist the maintenance of a discharge on the surfaces participating inthe process. One of these special measures consists in carrying out astarting process, on completion of which the stationary condition of thedischarge is attained, which condition takes place with predeterminedenergy transformation and predetermined division of output in respect tothe voltage carrying surfaces and can be maintained in continuousoperation, the energy transformation and the specific output notexceeding predetermined maximum values on any of the surfaces notparticipating in the process. The method is in no way limited todefinite arrangements of the electrodes and to definite shapes of thesurfaces participating in the process and can be used practically in allcases that may occur, as long as the arrangement is carried out asrequired by each case. In any event, however, the figures, usedhereinafter to explain the method, are only diagrammatically reproducedexamples of suitable arrangements and discharge receptacles. As isdescribed below, the carrying out of a process according to the presentinvention requires that it be carefully planned in accordance with theresults desired and being based on certain rules described hereinafter.

The discharge receptacle shown in FIGURE 1 for carrying out such aprocess is adapted to operate on a voltage of constant polarity, but notnecessarily of constant amplitude. The same comprises the removableupper part 1 and the bottom part 2, both preferably consisting of anelectrically conducting material such as metal. The parts 1 and 2 areconnected together in a gas-tight manner, and a gas atmosphere of anypressure and any composition may be produced inside through the gassuction pipe 3 and gas supply pipe 4. The upper part 1 is provided withan insulated lead-in 5, which in this case represents the anodeconnection, and a corresponding insulated lead-in 6 is provided in thebottom part 2, the same acting as a cathode connection; both lead-insare built in a gas-tight manner in the corresponding walls 1 and 2. Thelead-in connection 6 sup-ports, by means of suitably formed holders 7the article to be treated in the technical process; in this case, for1nstance, the metal workpiece 8. Opposite the latter there is providedan electrode 9, which is secured to the leadin connection 5 andrepresents the anode, but which should itself not participate in thetechnical process to be carried out. The problem now is, to limit thepowerful glow discharge, indicated in dotted lines by 10 in FIGURE 1, asmuch as possible to the outer surfaces of the workpieces 8 participatingin the process, and to attain there a predetermined value of the energytransformation for a predetermined specific output, without the othervoltage carrying parts, viz. the inside of the lead-in connections 5 and6, the holders 7 and the electrode 9, showing any glow discharge impactexceedlng the admissible maximum. Also the inner walls of the receptacleparts 1 and 2 should be as free as possible 6 from such disturbing glowdischarges and energy losses resulting therefrom.

The same problem arises also in connection with alternating currentoperation of such a discharge receptacle 1, 2, but, as shown in FIGURE2, in this case there is no longer any difference between the anode andthe cathode, for which reason now two workpieces 8a, 8b can be subjectedsimultaneously to the desired process, which workpieces are secured tothe one and the other holder 7 and 9 respectively and are connected withthe lead-in connections 6 and 5 respectively. The powerful glowdischarge 10a and 10b should be limited as much as possible to the otherouter surfaces of the workpieces 8a and 8b participating in the process.

This problemquite unsolvable in the glow discharge technique hithertoknown, as regards the outputs required for technical purposes-is onlysolvable when the desired end state of the gas discharge is producedthrough the starting process hereinafter described. However, in thewidely differing technical processes used in actual practice and withthe different forms of workpieces as well as the possible reactions ofthe surfaces that are participating in the process, etc. a carefulpreliminary planning of the required process is understandablynecessary. As regards discharge receptacles similar to those shown inFIGURES 1 and 2 with electrically conducting walls at least on theinternal sides, the following rules have to be taken into considerationfor the planning of the desired process, the said technical valueshaving to be ascertained, if necessary, by preliminary tests.

Gas pressura.'l'he required minimum pressure is given by the stipulationthat as regards the desired specific output on the surfacesparticipating in the process, the same are always completely anduniformly covered by the powerful glow discharge.

The pressure must not, under any circumstances, fall below that minimumvalue at which the less powerful glow discharges arising at the relievedvoltage carrying parts, and more particularly at the lead-inconnections, do not exceed the maximum impact of the discharge for theseparts.

Gas atmosphere-The composition of the gas atmosphere comprisingindividual gas components is determined by the kind of process that iscarried out. According to the amounts of the individual gas componentswhich are consumed or freely generated during the process, theirreplacement or removal is effected by a suitable gas supply and gassuction, the prescribed gas pressure in the discharge receptacle beingmaintained.

Arrangement of eIectr0des.-The distance between the energy-favoredsurface participating in the process, and the correspondingcounterelectrodes (which in the case of alternating current operationmay also be surfaces participating in the process), should be small incomparison with the area of the surfaces participating in the process.This distance will hereinafter be referred to as electrode distance.

Compared with the electrode distance, the distance between all thevoltage carrying parts and the walls of the receptacle should be asgreat as possible.

As regards the electrode distance, however, there is a limitingstipulation, namely, that the same must not be made smaller than doublethe thickness of the cathode drop space of the glow discharge in theintermediate space of the electrodes.

Shape of the surfaces-The surfaces participating in the process aregenerally given; however, it is advantageous to make the same as largeas possible as compared with the sum of the surfaces of allenergy-relieved voltage carrying parts. If, for instance as in the caseof hollow spaces, surfaces of the same potential lie opposite eachother, the so-called hollow cathode effect has to be taken intoconsideration, the same giving a higher energy yield of the glowdischarge, as soon as the thickness of the cathode drop space exceedsapproximately one-fourth the distance between these surfaces.

Size of receptacle.--The minimum size is determined by the required walldistances of the voltage carrying parts, as well as the size of thesurfaces participating in the process. On the other hand, theabove-mentioned stipulation of a surface area as small as possible forall surfaces not participating in the process determines the maximumadmissible size of the discharge receptacle.

Behavior of the surfaces participating in the process.- In addition tothe area and shape of the surfaces participating in the process, theirbehavior with respect to the conditions of the starting and end stateshas to be taken into consideration. Especially, their capacity ofemission at the stipulated temperature of the process, and under theintensive ion bombardment, is important, as well as the gas delivery tobe expected, evaporation, gas absorption and other properties of thematerial.

In the above-mentioned starting process, the gas pressure and theelectrode voltage are so adjusted at the beginning so as to be adaptedto the electrode arrangement provided according to the above-mentionedrules, and to the required initial temperature, and effect a glowdischarge which is of any desired extent. The electrode voltage and thegas pressure are advantageously so chosen that in the initial phase ofthe starting process all voltage carrying parts are covered by a glowlayer. The insulating lead-in connections and 6 are sensitive to thedetrimental effects of even the glow discharges of little power producedduring the starting process and are provided with means, hereinafterdescribed, capable of protecting them.

The phase of the starting process wherein all voltage carrying parts arecovered with a glow discharge is continued until the defects of thesurface layers are removed. After removal of all or any imperfections inthe surface layer, the glow covers the voltage carrying surfaceuniformly. Now, the gas pressure and, generally, also the electrodevoltage are continuously increased whereby the specific output of thedischarge at the surfaces participating in the process is increased andthe energy transformation is increased in steps. However, the specificoutput of the glow discharges at the surfaces not participating in theactual process is not increased to the same extent, and in certaincircumstances is even reduced, so that the energy transformation ismainly limited to the desired surfaces, which, therefore, are heated upgradually, approaching the temperature required for the metallurgical orchemical process to be carried out. This end state of the discharge isreached after a starting period of time, which is characteristic foreach process and the kind and size of the participating surfaces orlayers of gas, whereby a division of output is attained and the powerfulglow discharge is largely concentrated upon the surfaces participatingin the process, showing there a predetermined value of specific outputand of energy transformation, whilst at all the other voltage carryingparts, especially at the lead-in connections, the discharge impact doesnot exceed a given maximum. If desired, the glow discharge during thestarting process may be stabilized by the insertion of a seriesimpedance, for instance an inductive impedance, in the supply circuit ofthe electrodes. Instead thereof, or additionally thereto, a practicallyinertia-free control of the electrode voltage may be provided, which,upon a predetermined adjustable maximum current being exceeded, or upondropping under and adjustable minimum voltage, effects a voltage drop ofshort duration to a predetermined value, or switches off the voltagealtogether. The purpose of both these measures is to avoid aninadmissible strong local heating of individual surfaces when greatirregularities suddenly arise in the surface layer, as for instance inthe case of gas eruptions.

The starting process must always begin with a smaller energytransformation and lower specific output than is provided for the endstate aimed at; that is, the output capacity of the dischargereceptacle, measured at the lead-in connections, should not amount tomore than about 50% of that of the end state, or it should even be muchlower. It is generally advisable to begin the starting process with areduced electrode voltage as well as with a reduced gas pressure, butthis is not absolutely necessary. In some instances, and in the case ofspecial arrangements of electrodes or processes, the gas pressure mayalso be the same at the beginning as at the later end state, and onlythe electrode voltage be reduced, or the electrode voltage correspondingto the end state be applied already in the starting phase and the gaspressure be correspondingly reduced. On completion of the startingprocess, if the process has been correctly planned in accordance withthe above-mentioned rules, the temperature and constitution of thesurfaces participating in the process are such as to ensure stable,continuous operation. If desired, the discharge receptacle may now beput out of operation even for short periods of time, for instance a fewminutes, and then immediately put into operation again with the fullpower. The end state that is reached is characterized by the fact thatthe condition of the gas enables a powerful glow discharge to take placeonly in the immediate proximity of the surfaces participating in theprocess and the thickness of the cathode drop space is always muchsmaller than the spatial distance between the surfaces participating inthe process and the next counter-electrode. The thin gas layercorresponding ap proximately to the thickness of the cathode drop space,and the confining surfaces play the main part in the energytransformation; this is true of all gases that may be used. Naturally,the thickness of the effective layer of gas is also determined by thecomposition of the gaseous atmosphere and the gas pressure. The gasconsumption involved in a process wherein gases react together as wellas in the case of most applications for ,metallurgical processes, has tobe compensated by a continuous supply of gas to the internal space,under the prescribed gas pressure which has to be maintained.

By following the rules described above and by means of the stratingprocess, it is possible, for instance, to treat workpieces in whichsurfaces up to 25,000 sq. cm. and more, participate in the process. Forinstance, the behavior of such a glow discharge with an energytransformation of 17,000 watts in the end state is shown by the curve Aof the current/voltage characteristic according to FIGURE 4. As alreadymentionedflrhe end state of the discharge can be interrupted for a shorttime without any detriment, so that it is possible to obtain such acharacteristic. The curve A was ascertained in connection with aworkpiece having a total surface of F =4,000 sq. cm. participating inthe process, at a pressure of P=5 .7 mm. Hg and a temperature of T=505C. of the relevant surfaces in the stationary end condition. If theterminal voltage U is continuously increased up to the value B, acomplete covering of the glow of the parts to which a voltage isapplied, is obtained, and with further increase of from U there isobtained an increase in the specific output of the glow discharge,mainly only on the desired surfaces. Here, the function 1: (U) does notindicate any of the usual instability like the characteristi'cs ofnormal glow discharges shown in FIG. 15 (beyond a current of 0.5 to 1amp) and thus reveals an entirely different behavior from that of theglow discharges hitherto known. The specific output is about 4.2 wattsper sq. cm. surface for a current of I =30 amps which has to bemaintained during continuous operation. This, of course, is only thespecific output for the surfaces of F=4,000 sq. cm. participating in theprocess; it is less on all the other parts to which a voltage isapplied, more particularly at the lead-in connectors.

The characteristic C represented in FIG. 4 relates to the treatment of aworkpiece with a specific output of 5.3 watts per sq. cm. on thesurfaces participating in the process, at a higher gas pressure ofP=10.5 mm. Hg. In the characteristic, there appears at the value D ofthe electrode voltage U a slight indication of the point of instabilitywhich is strongly defined in the usual glow discharges. In bothexamples, this specific output is reduced during continuous operation toa desirable middle Value by the lowering cycle to be hereinafterdescribed.

The fact that the glow discharge according to the present invention is anew kind of glow discharge is evident from the comparison of the wellknown current/ voltage characteristic 65-67 of FIG. 15 with thecharacteristic 66 which is identical with curve A of FIG. 4, but plottedin the same coordinates as the characteristic 65-67. To prove thedifferent behavior of the two kinds of glow dicharges shown in FIG. 15it is sufiicient to calculate the inherent impedance or resistance R, ofthe discharge.

The current/voltage characteristic 65-67 of FIG. 15 which is consideredto be characteristic for the well known low-intensity glow discharges,was evaluated for eight ditierent points for the current I between .2and 1.0 amperes, as per the following table:

Evaluation of the curve 65-67, FIG. 1

I Amp. U Volts Ra U/I N Watts dR Iii/R -100% Ohms For each of the eightpositions, the internal resistance R,: U/I and the power N wascalculated. In addition, the charge in resistance dR, between theindividual points was determined and, moreover, the resistance ratio R/R in percent, the reference value being the resistance R =2,500 ohms ata cu-rrent 1:.2 amps. Correspondingly, the two characteristic curves Aand C in FIG. 4 were evaluated in accordance with the table below:

Evaluation of the curves A and C, FIG. 2

I I R; N dRi Ri/R '100% Amp. Volts Ohms Watts The reference resistance Rwas taken as the value R, at a current of I=.4 amps, i.e. R =375 ohmsfor curve A and R =750 ohms for curve C.

If the shape of the inherent discharge resistance R, of the threecharacteristics is plotted in dependance of the discharge current I, thecurves according to FIG. 16 are obtained, wherein:

Curve 1 indicates the inherent resistance R, for the hitherto known glowdischarge according to the characteristic 65-67 in FIG. 15;

Curve 2 indicates the inherent resistance R, for the characteristic Aaccording to FIG. 4, i.e. for an area of 4,000 sq. cm. at a gas pressureof 5.7 mm. Hg;

Curve 3 indicates the inherent resistance R, for the characteristic Caccording to FIG. 4, i.e. for an area of 650 sq. cm. and a gas pressureof 10.5 mm. Hg.

FIG. 16 shows that the inherent resistance R, for the known glowdischarge displays values between 2,500 and about 10,000 ohms. On theother hand, the curves 2 and 3 relating to the high-current glowdischarge at an operating point according to the present inventionreveal values of the inherent discharge resistance no higher than about4-00 to 700 ohms. The difference in the magnitude of the inherentresistance R, of the two types of discharge may easily reside in thefact that the current/voltage characteristic 65-67 in FIG. 15 wasdetermined in a discharge chamber having relatively small electrodesurfaces, the shape of the cuves in FIG. 16, however, shows that theinherent resistance at the usuable range of lowintensity dischargesincreases with increasing discharge current and voltage, while itdecreases with increasing discharge current and voltage in the case ofhigh-current glow discharges.

The values obtained from the above tables in respect of the inherentresistance R are shown in kw. in FIG. 17 plotted against the power Nsupplied. As in FIG. 16, the curve 1 in the figure corresponds to thelow-intensity glow discharge, while the curves 2 and 3 indicate thebehavior of the high-current glow discharge. It may be seen thatrelatively high power up to about 4 kw. can be obtained also in ahitherto known glow discharge according to the curve 65-67 in FIG. 15,i.e. a power as employed in the processing of a work having 650 sq. cm.according to the characteristic C in FIG. 4. Again, it is clearlyvisible that the inherent resistance R, in low-intensity glow dischargesincreases along with the increasing power employed in the dischargechamber, while it becom% smaller with high current glow discharges atincreasing power consumption.

The fact that the curve 1 of the hitherto known lowintensity glowdischarges shows an increase in FIGS. 16 and 17 while the curves 2 and 3for the new high-current glow discharges decline comparatively steeply,cannot be explained by the possibility that the characteristic 65-67 inFIG. 15 was obtained with much smaller electrode surfaces than thecurves A and C according to FIG. 4 compared with it. If the resistanceof a discharge chamber increases when the current or the power consumedincreases, this tendency may not be altered even by parallel arrangementof a plurality of similar discharge chambers; accordingly, the sametendency must apply to larger electrode surfaces; the behavior of theinherent resistance R, in dependence of the discharge current and,respectively, of the power consumed can therefore hardly be determinedby the electrode surfaces used.

In order to make still clearer the different behavior of the inherentresistance R, with low-intensity and highcurrent glow discharges, FIG.18 shows the percentage change of the resistance R, in dependence on thedischarge current I and, respectively, of the power N consumed, thereference point for the low-intensity glow discharges (curves 1) beingselected as the resistance value R with a current of .2 ampere, and thevalue of R with a current of .4 ampere for high-current glow discharges(curves 2 and 3). The full-line curves in FIG. 18 show the percentagechange of the resistance in dependence on the discharge current I and itcan clearly be seen that with the known low-intensity glow dischargesaccording to curve 1 the inherent resistance will rise from 100% to 450%when the current increases, while it drops from 100% to -10% with risingcurrent in the case of high-current glow discharges.

A similar picture is presented by the curves in FIG. 18 shown in brokenlines, which represent the percentage change of the inherent resistanceR, in dependence on the power consumed in kw. With an increase of thepower consumed of about .1 kw. to 4 kw., the inherent resistance willrise from 100% to about 450% with low-intensity glow discharges, whitethe discharge gap resistance in high-current glow discharges drops fromabout 100% to about Particular mention is made of the fact that theassociated curves 2 and 3 showing the behavior of the inherentresistance with high-current glow discharges are very close in FIG. 18although an electrode surface was employed for the curve 2 of over tentimes the size as for the curve 3, while the gas pressure relative tocurve 3 was twice as high as that used for curve 2. It is thus obviousthat the percentage change of the inherent resistance in dependence onthe discharge current and, respectively, on the power consumed, is onlyslightly dependent on the electrode surface in high-current glowdischarges.

In view of the above results of calculation, it has been proved that atleast at currents beyond 1 amp. there exists a fundamental difference inthe behavior of the inherent resistance between hitherto knownlow-intensity glow discharges and the high-current low-voltage glowdischarges according to the present invention.

The rules for approaching an operation point showing the behavior of theabove described new kind of glow discharge comprise the steps regulatingoperating voltage and the gas pressure in the discharge chamber first tovalues sufficient to produce a discharge of less current strength andlower energy density than at the said operating point, regulating thenboth voltage and pressure to higher values thereby altering the ratiovoltage/current, i.e. the inherent resistance of the discharge chamberand proceed this raising to a current beyond 1 amp. and to an operatingpoint in the environs of which the inherent resistance decreases withincreasing current and voltage. The inherent resistance or impedance ofthe discharge being always less than 1000 ohms. The voltage preferablyis not higher than 1500 volts.

It is pointed out that regulating of voltage or pressure or both can becarried out in a step by step manner or, if desired, in such small stepsthat a more or less con tinuous increasing is obtained.

The method according to the invention is, if the electrodes in thedischarge vessel are appropriately arranged, suitable for carrying outmanifold metallurgical processes and more particularly for treatingsurfaces of metal workpieces, for instance for nitriding steel surfaces.It is possible to treat even very complicated surfaces provided withrecesses and holes and, if desired, to obtain the same specific outputat all the points of the energy favored surfaces. Naturally, for otherreasons, a continuous characteristic according to FIGURE 4 cannot beexpected. For instance, it is possible to attain a division of outputsuch that the inner walls of steel tubes with a diameter of between 0.5and 2 cm. are energy-favored, it being then possible to obtain specificoutputs of 0.5 to 6 watts per sq. cm. surface, with an electrode voltageof the order of 400 to 600 volts and a gas pressure of 2 to mm. Hg. Alsoin the case of holes of larger diameter, of 2 to 10 cm., correspondingprocesses can be carried out on the inner walls by using approximatelythe same electrical data. The gas pressure to be used is always lower inthe case of bores of smaller diameters and higher in the case of largerdiameters. In the case of bores having a length smaller than about 80times the diameter, it is possible, by a suitable choice of electrodevoltage and gas pressure, to make the powerful glow discharge, accordingto the division of the output, cover the whole innerwall of the bore ina largely uniform manner. On the other hand, when the ratio of thediameter to the length of the bore is greater than 1:80, it ispreferable to provide, for instance, a wire-like auxiliary electrodealong the axis of the bore. According to the type of process carriedout, this axial counter-electrode may constitute the anode if directcurrent voltage is used for the supply of the discharge receptacle andno glow discharge is shown on the surface acting as an additional supplyof energy, or if an alternating current is used, its surfaceparticipates in the process, which, owing to its small extension causesstrong heating and secondary processes, which may be desired, as forinstance evaporation of the wire and deposition upon the inner wall ofthe bore (diffusion of chromium, tungsten, etc.). 7

When metallurgical processes have to be carried out 011 the surfaces ofworkpieces, a very definite temperature has generally to be maintainedwithin narrow tolerances as regards the surfaces participating in theprocess. On the other hand, to carry out the desired reactioneffectively, it is frequently necessary to get as high as possible aspecific output at the corresponding surfaces. However, since thesurface temperature depends upon the dispersal of the heat, which variesaccording to the workpieces, it cannot be expected that the hightransformation of energy, desired because of the reaction, shouldcorrespond to the supply of energy required in order to maintain adefinite surface temperature. This applies especially to the treatmentof the inner wall of tubes, since in this case, although the inner andouter wall may be surfaces participating in the process, viz. may serveas an energy supply, the inner wall contributes but very little to thesupply of heat. This difiiculty is avoided according to the invention bythe fact that the electrode voltage is reduced from the nominal value ofthe end state of the discharge, for a short period of time, to a lowervalue and this drop is repeated in a predetermined time cycle. l-Iere,however, at least during the interval with normal voltage, the divisionof the output is maintained.

For instance, as regards the same process, the starting of which isrepresented in FIGURE 3, FIGURE 5 shows the lowering cycle of theelectrode voltage, and the output N in dependence upon the time t, aworking interval of 0.5 second following a lowering interval of 1.5seconds. By a suitable choice of the remaining energy transformationduring the lowering interval and by changing the ratio between workingand lowering intervals, a suitable temperoral middle value of the energytransformation may be adjusted so as to maintain a predetermined surfacetemperature, in this case for instance 1.5 watts per sq. cm., and inspite of the high specific output required for carrying out the desiredprocess, taking into consideration the heat balance, can be maintainedduring the working intervals. The cyclic drop in the output has provedadvantageous, especially in the metallurgical treatment of bores. Inthis case, the cyclic output drop has additionally a valuable secondaryeffect, namely the gas pressure, too, changes in the bore with thefluctuations in the output, which results in a gas exchange in the bore,if the working and lowering intervals are suitably chosen. However, sucha gas exchange is required, in order to prevent the gas volume withinthe bore from being impoverished of substances which are consumed in therespective process.

The method according to the invention can be used with advantage for adivision of output in favor of large surfaces of individual workpieces,as well as for simultaneous energy-favoring of small surfaces on anumber of workpieces and, by the proper arrangement of the individualworkpiece, if desired, by using auxiliary electrodes, it is possible tolimit the powerful glow discharge to the surfaces of the individualworkpieces participating in the process. When the discharge receptaclehas applied to it a voltage of constant polarity, all the workpieces areconnected as a cathode. When a supply of alternating current-voltage isused, the workpieces with the surfaces to be favored may be connected ingroups and be connected to the individual phases of the source ofalternating current, thus, for instance, in three groups when the supplyis effected by means of a three phase alternating current voltage;preferably, the individual workpiece surfaces may in this case be soconnected that adjacent pieces lie on different phases.

Of course, the method of dividing output according to the invention is'in no way limited to the carrying out of metallurgical processes, butmay be also used with advantage for producing chemical reactions. Inparticular, the gas layer, which is in the immediate proximity of thesurfaces participating in the process and is especially effectivebecause of the presence of ionized and atomic constituents of thegaseous atmosphere can be favored thereby with respect to the divisionof energy. Such constituents co-operate very actively in the reactionsfor carrying out processes between gases, as well as between gases andsolid or liquid substances, and are produced in increased quantities asa result of the division of output. Furthermore, the high kinetic energyof the gas particles in this layer is used for instance for heatingsolid substances in order to produce reactions with the gaseousatmosphere. In connection with such chemical processes it should benoted, that the objects to be treated always must conduct the electriccurrent but can show a nonmetallic nature. For example, the oxide ofaluminum is non-metallic but is a conductor at elevated temperatures,such as l'0 C. Also semi-conductor material can be treated according tothe invention.

However, the energy favoring of the surfaces participating in theprocess themselves can also be advantageous for the chemical influencingof said surfaces. For instance, it is possible to reduce ores in ahydrogen atmosphere, the division of output in the process beingcontrolled by the above-mentioned rules, so that it is possible tomaster it to a great extent, as regards both the speed of reaction andthe temperature that occurs, even when large amounts of gaseous reactionproducts are produced and have to be led away. In the treatment of oxideores as an energy-favored Surface in a hydrogen atmosphere the reductionproduces water vapors in addition to oxygen, and according to the kindof admixtures used also other gaseous substances, even during thestarting process. After the end state is reached, the reaction iscompleted and, if desired, the end product may be melted by furtherdividing the output while increasing the electricalenergytransformation. According to the kind of substances to be treated, theenergy favoring is effected in a solid condition thereof, or, if desiredand advantageous, only after said substances have been melted, e.g. forthe purpose of reducing or oxidizing them. Moreover, other physical orchemical processes, such as diffusion processes, spraying of electrodesurfaces, producing and using catalytic actions of finely dividedsubstances, hydrations, etc. may be carried out by a division of outputin favor of the surfaces participating in the processes, possiblyutilizing the large kinetic energy of the gas particles in the gas layeradjacent to these surfaces.

In all such chemical and chemico-physical processes, the same rules comeinto consideration with regard to the connection of the surfaces to befavored with respect to the energy, as in the case of the treatment ofworkpieces above mentioned. Accordingly, these surfaces, that is to say,the substances to be treated, are connected to the cathode in the caseof a direct current supply, whilst in the case of an alternating currentsupply, such a reaction surface is supplied by a phase thereof. Thedivision of the output of the powerful glow discharge in favor of thesurfaces participating in the process can thereby be attained in thesame way as described with respect to the treatment of workpieces.

The apparatus for carrying out the process according to the inventionpreferably consists at least partly and especially at the electrodelead-in connections, of a vessel with electrically conducting walls, forinstance, as is diagrammatically illustrated in FIGURES 1 and 2.However, the invention is not limited thereto. For instance, FIGURE 6shows a receptacle construction in which the two attachable ends 12 and13 are of electrically conducting material, but separated from eachother by a cylindrical intermediate member 14 of insulating material.The lead-ins 5 and 6 are provided in the ends 12 and 13 respectively.The shape of the receptacle can, of course, be adapted to a large extentto the shape of the article to be treated. It appears easy to avoiddifficulties that may arise by using walls of insulating material in theproximity of the lead-in connections. However, this is not possible inpractice since insulating inner walls become very quickly electricallyconductive during operation (spraying etc.).

One embodiment, given by way of example, of a discharge receptacle andcontrolling means for carrying out a tempering process, wherein theinner wall of a steel tube is favored in respect of energy, is showndiagrammatically in FIGURE 7. The discharge receptacle consists of anelongated cylindrical chamber 15 closed in a gas-tight manner by meansof an upper and lower cover 16 and 17 respectively. The walls of thechamber 15 and the two covers 16 and 17 are double-walled in order toallow for a fiow of cooling water. Lead-in connections 18 and 19 projectinto the inner space through the two covers 16 and 17 respectively.These lead-in connections 18 and 19 are also water cooled and thecooling water for the whole of the discharge receptacle is supplied fromthe pipe 2%, through the lead-in connection 19, the pipe 21 to thedouble-walled cover 17, through the latter and the pipe 22 from below,into the cooling jacket of the chamber 15, and leaves the same at thetop through the pipe 23, flows through the pipe 23, flows through thedouble-walled cover 16 and through the pipe 24 to the lead-in connection18 and therefrom to the discharge pipe 25. In the interior of thedischarge receptacle there is, for instance, suspended at the upperlead-in connection 18, through the stirrup 26, a steel tube 27, thewalls of which, particularly the inner wall, are the surfacesparticipating in the process. Since the bore of the tube 27 to betreated has to have a length greater than times its diameter, there isprovided a thin rod 28 acting as a counter-electrode, the same beingsecured to the lower lead-in connection 19 and projecting freely out ofthe bore along the axis of the tube, or it may be supported at its upperend by means of a further lead-in connection, on the receptacle wall. Inthis way, the tube 27 is connected as one electrode to the connection 29and the rod 28 is connected to the connection 39 as the other electrode.The temperature measurement of the workpiece 27 is effected through aninspection window 31 by means of a radiation pyrometer 32.

The interior of the discharge receptacle may be evacuated to therequired extent through the gas suction pipe 33 and the valve 35, bymeans of a suitable pump arrangement 34. A low-pressure gauge 36 isprovided to indicate the gas pressure, and it so controls the gas inletvalve 38 through the device 37 that the predetermined gas pressure ismaintained in the interior of the chamber 15. The composition of thegaseous atmosphere in the interior of the discharge receptacle may becontrolled by the supply of gas through the pipe 39, which is fed,through the valve 38 controlling the admitted amount, for instance fromtwo gas cylinders 40 and 41, through the pressure reducing valves 42 and43 and the stop valves 44 and 45 respectively. The shown installationcomprising two gas cylinders 40 and 41 is adapted for instance, for theuse of an inert gas, for example krypton, and the admixture of anothergas, for instance, nitrogen for the gaseous atmosphere in the dischargereceptacle.

The electrode treminals 29 and 30 are connected to the negative andpositive poles of a source of direct current 46 respectively, and aseries impedance 47 is inserted in the lead to the connection 29, whichimpedance can be shortcircuited by a switch 48. The source of directcurrent 46 is in this case, for instance, a rectifier which is suppliedat the connection 49 with a single-phase alternating current, and can becontrolled by a regulating device with regard to the voltage that isapplied. The regulating device 50 is actuated, on the one hand, by apredictor 51 and, on the other hand, by the measured voltage which isproportional to the temperature indicated by the pyrometer 32. Thesource of direct current 46 may be such that the positive as well as thenegative poles are not connected to earth, and that the dischargereceptacle itself is earthed or the positive connection 30 together withthe discharge vessel may be earthed.

During the starting process the discharge receptacle is preferablyhand-controlled. In this case, the switch 48 is open so that theimpedance 47 lies in the electrode circuit to limit the current of theglow discharge. The control device 37 for obtaining the desired gaspressure may also be hand-controlled.

To approach the desired operating point at the discharge characteristic,the gas pressure at the beginning was about 2.5 mm. Hg and the dischargeoperated with a voltage of about 350 volts D.C. producing a current ofabout 7 amps. After a time interval of 10 minutes the pressure wasincreased to 4.5 mm. Hg and the discharge operated with 420 volts and 12amps. during a time interval of one-half hour. The inherent resistanceor impedance of the discharge chamber decreased between the twooperating points from about 50 ohms to about 35 ohms. The temperature ofthe gun barrel was at the end of the second interval about 480 C. In thedescribed example a nitriding of the inner wall of the barrel 27 was tobe carried out at a temperature of 505 C. so that the energy density hadto be further increased. The gas pressure was therefore raised to 5.7mm. Hg and the voltage increased during a time interval of 30 minutesstep by step to 465 volts and an operating point with the desiredtemperature reached at a current of about 15 amps. or a powerconsumption of about 7 kw. The mean value of the energy density wasabout 1.75 watts per sq. cm. corresponding to an area of the outside andinside surface of the gun barrel of about 4000 sq. cm.; the effectiveenergy density was higher on the inside surface and lower at the outsidesurface. The characteristic of the discharge chamber is shown as curve Ain FIG. 4. Also in the environs of the operating point characterizingthe end state of the approaching process the inherent resistance orimpedance showed a decrease with increasing current and voltage.

When the starting process is completed, the series impedance 47 isshort-circuited and the electrode voltage is automatically maintained atsuch a value that it will ensure the predetermined temperature of thesurfaces participating in the process, for which purpose the temperaturemeasuring device 32 correspondingly controls the regulating device 50.At the same time the predictor 51 eifects the lowering cycle of theelectrode voltage through the same regulating device 50 (see forinstance FIGURE 5).

As found by experience, the carrying out of technical processes of thiskind in a reliable manner by dividing the output of the powerful glowdischarge requires special measures as regards the insulated lead-inconnections 18 and 19, in order to avoid the accumulation of deposits ofmaterials and the detrimental effects of glow phenomena, which stilloccur, despite relieving, on the insulating parts. An embodiment of suchan insulated leadin connection is shown, by way of example, in axialsection in FIGURE 8, the same being incorporated in the double-walledupper cover 16. The middle conductor 52 with the fixing bolt 52aprojecting into the interior of the receptaclev and supporting theholder of the workpiece 26 (FIGURE 7) is so arranged as to allow forwater cooling, the water entering the pipe 23 and flowing out throughthe pipe 24. The middle conductor 52 is insulated from the cover'16 bymeans of the insulating members 53 and 54, which are pressed by means ofthe screw cap 55 against the corresponding abutting surfaces of thecover 16, ensuring on the one hand, a gas-tight closure, and one theother hand, enabling the whole of the lead-in connection to be easilydismounted. The middle conductor 52 is surrounded by a metal sleeve 56at the end directed towards the interior of the receptacle, which sleeveis partly screened by a metal cap 57 secured to the inner wall of thecover 16. The gap system shown in FIGURE 8 prevents a powerful glowdischarge and thereby incrcases the maximum admissible impact of thedischarge on the lead-in connection. The gap system .consists of theannular gap 58 between-the (grounded) cap 57 and the sleeve 56 to whicha voltage is aPPlied, the main gap 59 between the (grounded) wall 16 andthe sleeve 56, the transverse gap 60 between the sleeve 56 and theinsulator 54, and the annular gap 61 between the insulating member 54and the (grounded) wall 16. By preventing a powerful glow discharge bysuitably dimensioning the gap system and the water cooling, such aleadin connection can be used with perfect safety as regards thedischarge impacts, which can hardly be avoided in technical processes inspite of the energy relieving.

The division of output and the energy favoring of certain desiredsurfaces in a powerful glow discharge was clearly demonstrated in anarrangement including a molybdenum tube which was brought to annealingtemperature by the glow discharge. The molybdenum tube, about 8 mm. indiameter and 50 mm. long, formed one electrode, while the otherelectrode was a metal bolt located about 40 mm. away from the saidmolybdenum tube. This discharge distance was operated with analternating current of 700 volts and 50 cycles between the twoelectrodes in a hydrogen atmosphere having a pressure of 9 mm. Hg. Themolybdenum tube revealed in continuous operation a temperature ofapproximately 2000 C. and had on its outer surface an energy density ofsome 50 watts per sq. cm. It could be clearly seen that the output ofthe glow discharge was largely concentrated on the molybdenum tube, forthe latters supporting wires-despite the heat conduction from theincandescent tubewere no longer incandescent a short distance away fromthe molybdenum tube. It was also possible to observe a distin'ctdivision of output in favor of the molybdenum tube. on thecounter-electrode which, despite the powerful radiation heating from thewhite-hot molybdenum tube, emitted only a feeble glowand even that wasright at its front end.

In this example of a division of output in an electric glow dischargethe conditions were, of course, already extreme as a result of thepowerful total emission current which amounts to approximately 4milliamps per sq. cm. However, there was no change into an arcdischarge, nor were there any noticeable amounts of energy transformedon the other surfaces not participating in the annealing process. Itwould have been quite possible, if the pressure were increased, to raisethe temperature to approximately 2700 and thus to melt the molybdenumtube.

In the above example the geometrical arrangement of the electrodes wasin accordance with the above-mentioned rules. The distance of 40 mm.between the electrodes was relatively small in comparison with theinternal and external surfaces which were favored in respect to theenergy and have an area together of some 2800 sq. mm. On the other hand,the distance from the leading-in insulators was very much greater, as isthe distance from the receptacle walls. The sum of all the surfaceswhich were not energy-favored but carried voltage, that is to say, thesupporting wires and the bolt of the counter-electrode, was likewisesubstantially smaller than the energy-favored surface of the molybdenumtube.

t The possibility of a division of output in glow discharges at theelectrodes in a vessel having a reduced pressure, such that certainelectrode portions are preferred while other portions carrying the samevoltage are largely free from glow discharges, appears most unusual inthe light of generally known physical principles. It must be consideredthat these are glow discharges having an energy transformation of over 1kw., which is a type of operation in which, with the conventionalinsulating discharge vessels of small dimensions, the range of normalglow discharges should long have been exceeded and the complete coverageof all cathode surfaces expected with certainty. Prior to this inventionit was considered certain that an increase in power increased the rangecharacteristic of abnormal discharges would be reached. Accordingly itcould not be expected that with the energy transformation in theglow-discharge partial coverage of the energized electrodes could beobtained in a predetermined manner.

As explained above, the high energy transformation required inlarge-scale glow-discharge processes for the heating of the material tobe treated and of the surfaces participating in the process, involves arelatively high specific density of energy of usually above .3 watt/cm.at said surfaces. In order to avoid the necessity of raising theoperating voltage to unpractically high values above 1000 volts, theonly possibility is to increase gas pressure in the discharge vesselabove 1 mm. mercury.

If, for instance in the metal discharge vessel 101 which isdiagrammatically reproduced in FIG. 9 and has two insulated currentlead-ins 102 and 103, a hold-er 104 for the tube 105 whose surface is tobe treated, and a counter electrode 106, an operating voltage of some600 volts is applied at about 1 mm. Hg via the voltage source 107, aglow discharge is produced which covers all the cathodic structuralmembers, viz. the workpiece 105, the holder 104 and the internal lead108 of the current lead-in 102. Naturally the density of energy at theworkpiece 105 is still relatively small with this mode of operation andis suflicient at most to heat said workpiece to 100 to 200 C.

If a density of energy of e.g. 1.5 watt/cm. and a temperature of 500 C.of the workpiece is to be Obtained and maintained for an extendedperiod, the voltage must be correspondingly increased. Naturally thiscauses the density of energy at all cathodic parts, also at the internallead 108 of the current lead-in 2, to be increased, which is mostundesirable since the sealing means at such heavily loaded lead-ins mustusually be protected against elevated temperatures.

In the attempt to keep the high-energy glow discharge away from thecurrent lead-ins as far as possible, it was tried substantially toincrease the height of the discharge vessel 101 while retaining thearrangement of parts 103 through 106 and to connect the holder 104, bymeans of a thin rod, with the internal lead 108 of the current lead-in102, which was then substantially remote. Examination of suchexperimental arrangements revealed the astonishing fact that the longrod connection was only partially covered with glow discharges while itstop portion including the current lead-in 102 was entirely freetherefrom. The only partial coverage of very elongated electrodes withglow discharge proved to depend on pressure, and increases in pressureover and above 1 mm. mercury achieved an increasing concentration of theglow discharge on the tube 105 and the immediately adjacent space.However, this is by no means the effect known in glowdischarge tubes ofsmall dimensions where the electrodes are partially covered in theso-called stabilization range of the current/voltage characteristic. Inthe hitherto unknown eifect of partial glow-discharge coverage of veryextensive electrode arrangements, no corresponding characteristic wasfound, as already described in conjunction with FIGS. 4 and 5.

This new knowledge supplies a rule, which was practically tested on manydifferent electrode arrangements,

for the efiicient regulation of the discharge condition in such a mannerthat a division of output is achieved in favor of the surfacesassociated with the process while predetermined current carryingstructural members, particularly the current lead-ins are at leastpartially relieved. To this end, it is necessary to work with a gaspressure above a specific limiting pressure determined by the electrodearrangement, and to set the voltage at the value required to achieve thedesired density of energy or temperature, but in no event to exceed aspecific limiting voltage. In this way a state of discharge is createdin which the glow discharge just begins to withdraw from the currentlead-in. The material to be treated, i.e. tube in FIGS. 9 and 10, thenshows the desired specific density of energy while at least the internallead 108 of the lead-in 102 is relieved of energy. If desire, relief mayalso be extended to the holder 104. In operation, therefore, a gaspressure above the limiting gas pressure and a voltage below thelimiting voltage will preferably be set.

While according to the said rule a division of output is obtainable inall cases, precautions must be taken to produce the relief andconcentration of energy respectively exclusively at the locationsdesired. As investigations have revealed, the configuration of thecurrent carrying members termed electrode geometry determines thelocation where discharge is favored, where the concentration isobtained. The determining factor for the desired relief of the currentlead-in 102 in FIGS. 9 and 10 is, by the way of example, the arrangementand area of the counterelectrodes 106 associated with the tube 105 incomparison with the distance and areas at the point of entry of theinternal lead 108 of the current lead-in 102 into the reduced pressurechamber as compared to the adjacent metal parts connected to the{positive pole of the voltage source 107. This electrode geometry mustbe selected in such a manner that the capacity between tube 105-withoutholder 104and the members of the counterelectrode 106 extending alongits length is larger than the entire parallel capacity of both the otherstructural members situated in the reduced pressure chamber connectedwith tube 105 (parts 104 and 108) and the structural members at thepotential of the counterelectrodes 106 (bottom bend of 106 and internallead of 103). This parallel capacity is here determined by the seriescapacities between the housing 101 and the holder 104 together with theinternal lead 108 on the one hand, and the capacities between thehousing 101 and the counterelectrodes 106 together with the internallead of the lead-in 102 on the other. If the capacity value between thetube 105 and the counterelectrodes 106 exceeds that parallel capacity,the desired division of output, in which the lead-in 102 and theinternal lead 108 are relieved and energy transformation is concentratedon the process surfaces, i.e. tube 105, is obtained with simultaneousalteration of pressure and voltage and maintenance of the prescribeddensity of energy.

This capacity rule enables a suitable arrangement of electrodes to befound easily, which in turn renders possible a division of output infavor of the surfaces involved in the process. Naturally, this is onlyan abridged de scription of the geometric configuration of distances andareas and it is not implied that these capacities are actually operativeas capacitive imaginary components when the glow discharge is achieved.Actually, the impedance values of the individual portions of dischargespaces should probably be considered to be ohmic and to be resistances.This capacity conception, however, has proved useful as an aid indetermining the efiiciency of the structural arrangement.

If the capacity rule were not observed and the sum of parallelcapacities were to exceed the capacity between the tube 105 and thecounterelectrodes 106 in the discharge vessel and the arrangementaccording to FIGS.

159 9 and 10, a division of output would be obtainable but relief wouldtake place at the surfaces involved in the process, i.e. on tube 105instead of at the current lead-in as desired.

To decrease parallel capacity relative to process surface capacity, themetallic vessel 101 according to FIGS. 9 and 10 is grounded and notconnected to the voltage supply circuit, which causes the capacities ofthe two internal leads of the current lead-ins to be series-connectedagainst the housing 101. It is important, however, in this conception ofcapacity, that it is only the capacity of the internal lead 108projecting freely into the underpressure chamber that is of importance,not the self-capacity concentrated within the current lead-in itself,because the control of the glow discharge is not determined by surfaceson which no glow discharge whatever can occur; the dielectric constantof the insulation, too, is immaterial. FIG. 11 shows an embodiment ofsuch a current lead-in 102 in diagrammatic view, of which the internallead 108 is insulated from the metallic vessel 101 by means of aninsulator 109. In a known manner, insulator 109 is separated from theinternal lead 108 by a narrow annular gap 110 and by a similar annulargap 112 from the metallic insulator sleeve 111. The point of entry ofthe internal lead 108 into the reduced pressure chamber here designatedby the numeral 113 is where the internal lead 108 projects from the gapsystem of the current lead-in 102. The distance a of this point of entry113 of the internal lead 108 from the sleeve 111 is the determiningfactor for the capacity according to this specification, not theinternal capacity in the current lead-in 102. In the current lead-inaccording to FIG. 12, the sleeve 114 for the insulator 109 issubstantially closer to the point of entry 113 of the internal lead 108so that the distance a is considerably smaller and the capacity at thispoint substantially larger. With a view to perfect division of outputwithin the meaning of the present invention, such a structure must beavoided.

Besides the measure designed to reduce the capacities parallel to theprocess surfaces set forth in conjunction with FIGS. 9 to 11, FIG. 13shows that, as a further precaution, the discharge vessel can besubdivided into two half receptacles 116 and 117 which are insulatedfrom each other and are similar to the arrangements shown 'in FIG. 6.The insulating ring 118 should ensure a minimum capacity C1 between thetwo halves 116 and 117 "so that the parallel capacity comprising C2, C1and C3 in series connection to the process surfaces of the workpiece 119is as small as possible.

It may in some cases be desirable, as shown diagrammatically in FIG. 14,to provide a metallic discharge vessel 121 with covers formed ofinsulating material 122,123 in order to reduce the capacity of thecurrent lead-ins and to facilitate a concentration of the energytransformed on the material to be treated, by Way of example a melt 124.The material to be treated may be of metallic or non-metallic nature. aa

It should finally be pointed out that insulated metallic screensarranged in the reduced pressure chamber can prevent undesi'rable largeparallel capacities. By way of example, the outer metal holder 114 ofthe current leadin according to FIG. 12 can be insulated from thedischarge vessel 101 so that a screening effect is obtained. Also'theinternal lead projecting intothe reduced pressure chamber and theholders for the material to be treated can, if necessary, be screenedfrom the other metal parts. Such precautions may be of advantageparticularly in conjunction with narrow metal receptacles andunfavorable capacity conditions on the process surfaces.

Concentration of the energy transformation in the division of outputover the surfaces involved in the process can, except by reduction ofthe detrimental parallel capacity, be ensured by an increase in thecapacity of the process surfaces against the associatedcounterelectrodes.

By way of example, in the glow-discharge treatment of the tube in thedischarge vessel 101 according to FIGS. 9 and 10, there is thepossibility of using a counterelectrode having a large surface, such asthe metal cylinder 125 shown in FIG. 11 instead of the three rod-typecounterelectrodes at the distance d. An arrangement of the electrodesaccording to FIG. 13, too, causes the capacity between the workpiece 119and the cup-type counterelectrode to be increased.

The present methods of output division in a glow discharge onpredetermined current-carrying structural members can be applied to alltypes of operating voltages. By way of example, FIG. 9 shows the supplyby a directcurrent source 107 via a series resistance 126, whereperiodic short-circuiting of the latter by means of a switch 127controlled by the impulsing'means 128 creates a pulse-type energy supplywhich is of advantage when large discharge intensities are desired whilethe mean energy value per unit time determining temperature must not beexceeded. Naturally, some other means for pulse-type control can beprovided, by way of example a suitably controlled rectifier. Pure directcurrent, too, or rectified single or multi-phase alternating current maybe employed. Moreover, the supply may be effected by alternatingcurrent, such as 50 cps., in impulse control being possible ifnecessary.

According to the present invention, the right selection of the so-calleddischarge path resistance in the gas surroundings of the differentportions of voltage carrying at least intermittently negative parts isvery important. Referring to FIG. 9 the resistance alongv the dischargepath a from conductor 108 to the wall is greater than the analogueresistance between the object 105 to be treated and thecounter-electrode 106 along the path d. With a value of the dischargepath resistance in the surroundings of the upper portion of theconductor 108, high enough with respect to the analogue resistancea'tall other parts connected to the same conductor 108, the energy densityof the discharge on said portion can be'redu'ced by a predetermineddecreasing factor and if desired the said portion can be madesubstantially uncovered by the powerful glow discharge.

The decreasing of the energy density is very important, as abovementioned, at a junction between-metallic parts and an insulator, forexample the junction between the shoulder of the insulator plate 122carrying the dish with the melt 124 in an apparatus according to FIG.14. Such an unprotected junction is to be relieved with respect to theenergy density at the object to be treated, the melt 124, by decreasingthe discharge path resistance from the surroundings of the melt surfaceand the upper parts of the dish to the counter-electrode.

As already mentioned, the present measure, designed to relieve moreparticularly the cathodic lead-ins of energy, can be-applied at theoperating temperature ofthe process surfaces. It is, however, alsopossible, and advantageous in certain glow discharge processes, to setthe limit values -of gas pressure and voltage while the process surfacesare still cold, i.e. when their average temperature is between about 50and 100 C., and then to maintain said values when the temperature rises.According to the discharge space resistance then occurring, a completerelief of the current lead-ins is achieved when the discharge hasreached its end state, or else the initial state of relief is partlyreversed again.

The relief of certain voltage carrying and at least intermittentlycathodic structural members, in particular the current lead-ins, inaccordance with the method described above, must of course on noaccountbe carried so far in all cases that the energy transformationat thesemembers is approximately nil. On the contrary, a relief in the ratio of1:2 in favor of the process surfaces may already be of decisiveadvantage, depending on the design of the current lead-ins and themagnitudes of the energy transformation desired at the process surfaces.The

degree of relief to be achieved can be simply ascertained by comparingthe highest admissible specific glow discharge impact at the currentlead-ins, in watts per square centimetre of surface under impact, withthe prescribed specific energy transformation in watts per squarecentimetre of the process surfaces.

EXAMPLE 1 In an iron vessel 101 according to FIG. 9 having an innerdiameter of 450 mm. and current lead-ins of which the internal lead hada distance of a=70 mm. from its point of entry to the housing cover,steel tubes 105 of 530 mm. length, an outer diameter of 40 mm. and acylindrical bore of 7 mm. were treated. The three rods of 8 mm. diameterserving as counterelectrodes 106 were arranged at a radial distance d=35mm. from the tube surface and ran parallel to the tube along its entirelength. The distance from the tube surface to the inner wall of thevessel was b=205 mm., the total height of the interior of the vesselc=1200 mm.

The tube 105 was treated in a gesous atmosphere with a pressure of 11mm. mercury and a content of 75 percent by volume of hydrogen and 25percent by volume nitrogen. The steel of the tube contained percentchromium, 1 percent molybdenum, .27 percent carbon and .4 percentmanganese and had an initial hardness on the outside of 30:2 Rockwell C.

After a heating-up period of approximately two hours, treatment waseffected with a voltage changing between 480 volts and 395 volts inpulses with a current of 6.7 amps. for the high voltage and 2.7 amps.for the low voltage. The high tension was effective during an intervalof .3 second and followed by an interval of low tension lasting 1.4seconds so that the so-called impulse ratio of impulse vs. pause wasapproximately 1:5. With this operation, the glow discharge was obtainedat both the outside of the tube and in the bore, but the glow lightcoverage reached only the middle and the upper end of the holder 104respectively of tube 105 so that the internal lead 108 of the cathodiccurrent lead-in 102 was free from glow discharges. With this treatment,the tube reached an exterior temperature of 520 C. which was measuredthrough a window in the wall of the vessel by means of a radiationpyrometer.

After an initial period of approximately 2 hours and a period oftreatment under the above conditions of 27 hours, the voltage source wascut off and the tube left in the hydrogen-nitrogen atmosphere foranother 5 hours, and removed after cooling. Along the entire outside, avery uniform hardness of 50 HRcij HRc measured. Nitriding in ionbombardment achieved not only the increase in hardness desired by about28 HRc, but also a substantially greater uniformity of the degree ofhardness along the entire surface treated.

We claim:

1. In a process for establishing and maintaining a high current, lowvoltage glow discharge at a metal surface wherein said metal surface ispositioned in a closed vessel containing a gaseous atmosphere having apressure below atmospheric but equal to at least 1 mm. Hg, theconstructional parts of said vessel including a lead-in to said metalsurface which is insulated from the vessel wall, and an electrode, saidmetal surface and said electrode being connected with opposite poles ofa source of electrical potential, and wherein a glow discharge isproduced by an electric current between said electrode and said metalsurface at a potential difference sufficient to cause a glow dischargewithout arcing; the steps of; arranging said metal surface in spacedrelation to said constructional parts of said vessel so that theresistance to current flow in said atmosphere, when ionized, in theregion of said metal surface, is less than the corresponding resistancein all other regions in said vessel; initiating glow discharge withinsaid vessel to heat said metal surface to a temperature of at least 100C., said glow discharge covering portions of said constructional partswhich are negative relative to said electrode; and after heating saidmetal surface to said temperature, adjusting at least one of the factorsof voltage and gas pressure to cause current flow to increase in theregion of said metal surface and to diminish in other regions.

2. In a process for establishing and maintaining a high current, lowvoltage glow discharge at a metal surface wherein said metal surface ispositioned in a closed vessel containing a gaseous atmosphere having apressure below atmospheric but equal to at least 1 mm. Hg, theconstructional parts of said vessel including a lead-in to said metalsurface which is insulated from the vessel wall, and an electrode, saidmetal surface and said electrode being connected with opposite poles ofa source of electrical potential, and wherein a glow discharge isproduced by an electric currrent between said electrode and said metalsurface at a potential difference sufficient to cause a glow dischargewithout arcing; the steps of; arranging said metal surface in spacedrelation to said constructional parts of said vessel so that theresistance to current flow in said atmosphere, when ionized, in theregion of said metal surface, is less than the corresponding resistancein all other regions in said vessel; initiating glow discharge withinsaid vessel to heat said metal surface to a temperature of a least C.,said glow discharge covering portions of said constructional parts whichare negative relative to said electrode; and after heating said metalsurface to said temperature, adjusting at least one of the factors ofvoltage and gas pressure to cause current flow to increase in the regionof said metal surface and to diminish in other regions, to a point atwhich the overall resistance between said electrode and metal surfacedecreases with increasing voltage therebetween.

3. In a process for establishing and maintaining a high current, lowvoltage glow discharge at a metal surface wherein said metal surface ispositioned in a closed vessel containing a gaseous atmosphere having apressure below atmospheric but equal to at least 1 mm. Hg, theconstructional parts of said vessel including a lead-in to said metalsurface which is insulated from the vessel wall, and an electrode, saidmetal surface and said electrode being connected with opposite poles ofa source of electrical potential, and wherein a blow discharge isproduced by an electric current between said electrode and said metalsurface at a potential difference sufficient to cause a glow dischargewithout arcing; the steps of: arranging said metal surface in spacedrelation to said constructional parts of said vessel so that thecapacitance between said metal surface and said electrode is greaterthan the capacitance between said lead-in and any other constructionalpart; initiating glow discharge within said vessel to heat said metalsurface to a temperature of at least 100 C., said glow dischargecovering portions of said constructional parts which are negativerelative to said electrode; and after heating said metal surface to saidtemperature, adjusting at least one of the factors of voltage and gaspressure to cause current flow to increase in the region of said metalsurface and to diminish in other regions.

4. A process as defined in claim 3 wherein said glow discharge ismaintained at a potential difference, between said electrode and metalsurface, not exceeding 1000 volts.

5. A process as claimed in claim 1 wherein said metal surface and saidconstructional parts are arranged so that the distance between saidmetal surface and said electrode is small in comparison with the area ofsaid metal surface and the distance between all voltage carrying partsand said vessel wall is much larger than said distance between the metalsurface and the electrode.

6. A process as claimed in claim 1 wherein after said glow discharge hasbeen adjusted so as to increase the current flow in the region of saidmetal surface and to diminish the current flow in other regions, theenergy of transformation at said metal surface is regulated byperiodically lowering the voltage.

7.- Apparatus forestablishing and maintaining. a high current, lowvoltage glow dischargecomprising a closed metallic vessel; a memberhavinga conductive" surface positioned in said vessel; controllablemeans for maintaining a' gaseousatrnosphe'rein said vessel at a selectedsub-atmospheric pressure; ari electrical lead-in, extending into saidvessel'but insulated therefrom and connecting s'aid conductive surfaceto one pole of a source of potential; an electrode in said vessel,spaced from said con-- ductive surface, and connected to the oppositepole of said source of potential; said electrode and metal surface beingso spaced and arranged that the capacitance therebetween is greater thanthe capacitance between said lead-in-and' any other part of saidapparatus.

8. Apparatus" asdefined inclaim 7 wherein said lead-in includes acentral conductor having an-- insulating bushing therearound, saidbushing extending through an opening in a wall of said vessel andextending inwardly past the adjacent surface thereof; the inner portionof said vessel wall being radially spaced from said bushing to providean annular gap therebetween; and the inner portion of 245i saidi bushingbeing radially spaced? from saidi central: conductorto provide'asecondannula'r gap between said conductor'andsaidvesselwall;

9. Apparatus as defined in claim 7 wherein said electrode; at: leastpartially encloses. said? conductive surface to thereby increase thecapacity' between: said conductive surface and said electrode.

References Cited by the Examiner HYLAND BIZOT,-Priinary Examiner.

GEORGE N. WES'FBY, Examiner.

1. IN A PROCESS FOR ESTABLISHING AND MAINTAINING A HIGH CURRENT, LOWVOLTAGE GLOW DISCHARGE AT A METAL SURFACE WHEREIN SAID METAL SURFACE ISPOSITIONED IN A CLOSED VESSEL CONTAINING A GASEOUS ATMOSPHERE HAVING APRESSURE BELOW ATMOSPHERIC BUT EQUAL TO AT LEAST 1 MM. HG, THECONSTRUCTIONAL PARTS OF SAID VESSEL INCLUDING A LEAD-IN TO SAID METALSURFACE WHICH IS INSULATED FROM THE VESSEL WALL, AND AN ELECTRODE, SAIDMETAL SURFACE AND SAID ELECTRODE BEING CONNECTED WITH OPPOSITE POLES OFA SOURCE OF ELECTRICAL POTENTIAL, AND WHEREIN A GLOW DISCHARGE ISPRODUCED BY AN ELECTRIC CURRENT BETWEEN SAID ELECTRODE AND SAID METALSURFACE AT A POTENTIAL DIFFERENCE SUFFICIENT TO CAUSE A GLOW DISCHARGEWITHOUT ARCING; THE STEPS OF; ARRANGING SAID METAL SURFACE IN SPACEDRELATION TO SAID CONSTRUCTIONAL PARTS OF SAID VESSEL SO THAT THERESISTANCE TO CURRENT FLOW IN SAID ATMOSPHERE, WHEN IONIZED, IN THEREGION OF SAID METAL SURFACE, IS LESS THAN THE CORRESPONDING RESISTANCEIN ALL OTHER REGIONS IN SAID VESSEL; INITIATING GLOW DISCHARGE WITHINSAID VESSEL TO HEAT SAID METAL SURFACE TO A TEMPERATURE OF AT LEAST100*C., SAID GLOW DISCHARGE COVERING PORTIONS OF SAID CONSTRUCTIONALPARTS WHICH ARE NEGATIVE RELATIVE TO SAID ELECTRODE; AND AFTER HEATINGSAID METAL SURFACE TO SAID TEMPERATURE, ADJUSTING AT LEAST ONE OF THEFACTORS OF VOLTAGE AND GAS PRESSURE TO CAUSE CURRENT FLOW TO INCREASE INTHE REGION OF SAID METAL SURFACE AND TO DIMINISH IN OTHER REGIONS.